1. Field of the Invention
The present invention relates to a process and apparatus for the production of composite tubes. In particular, the present invention is directed to a process for the production of a composite tube including layers of plastic, metal and plastic. Specifically, the process and the apparatus of the present invention are directed to the production of an inner plastic tube, covered by a metal tube which is treated so as to reduce its diameter without causing the tube to elongate, followed by covering the metal tube with a layer of plastic.
2. Description of Background and Relevant Information
Composite tubes, made from concentrically disposed plastic metal and plastic tubes are generally available.
A German publication, DE 2 923 544, describes a process in which an intermediate layer of plastic material is coextruded onto an inner tube, made from plastic, followed by covering the multi-layer plastic tube with a metallic tube. The metallic tube, however, is subjected to a stretching and elongation operation.
A European publication, EP 0 024 220, describes a process in which two layers of plastic material are extruded within a metallic tube.
The previously described patent specifications are directed to processes which have attempted to solve the problem of producing tubes composed of two internal plastic layers located within an exterior metallic tube.
Another example of a conventional composite tube is described in EP 0 067 919.
In contrast, the process and apparatus of the present invention is directed to a solution to problems associated with the production of reinforced composite tubes having a metallic core which is concealed within plastic material. A composite tube in accordance with the present invention wherein an intermediate metallic tube is positioned between an inner and an outer plastic tube provides a tubular construction which ensures the requisite rigidity or semi-rigidity without the metal being exposed to the elements so as to minimize deterioration as a result of corrosion or damage by being subjected to external stresses.